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What are energy levels of an atom?

Answer Text:
What are Energy levels of an atom of an element:
- Are definite orbits in an atom that the electrons occupy.
- The energy levels are numbered 1, 2, 3……starting with the one closest to the nucleus.
- Electrons occupying the same energy level have approximately the same amount of energy.
- Each energy level can only accommodate a given maximum number of electrons.
Maximum number of electrons per energy level

Illustrations:
Hydrogen
- It has only one electron and thus this electron occupies the first energy level.
- Since the first energy level is not yet full, hydrogen does not have the second energy level;
- The electron arrangement of hydrogen is thus 1.
Helium:
- Helium is atomic number 2 and has only two electrons, which occupy the first energy level.
- The first energy level is thus completely full, but since there are no other r electrons lithium also has only one energy level;
- The electron arrangement is thus 2.
Chlorine:
- Chlorine has atomic number 17 and thus has 17 electrons;
- The first two electrons occupy the first energy level which is thus completely filled up;
- The remaining 15 electrons occupy the second energy level, which can however accommodate only 8 to be completely filled up;
- Thus the remaining 7 electrons move to the third energy level; which needs 8 to be completely filled up;
- Since the third energy level is not yet full chlorine does not have a fourth energy level;
- The electron arrangement is thus 2.8.7.